This invention relates to a process for producing an aromatic polyamide film excellent in tensile strength, tensile modulus of elasticity and transparency which is used in a protective film for lens, a recording magnetic tape and the like.
An aromatic polyamide, particularly all-para aromatic polyamide is one of the polymers having the highest tensile modulus of elasticity among the heretofore known polymers. A film obtained from the aromatic polyamide can be formed into a thin film having a thickness of several micrometers because of its very high tensile modulus of elasticity, and also has excellent heat resistance because of its very high melting point or decomposition temperature.
However, the aromatic polyamide is dissolved in only a special solvent having a high polarity such as conc. sulfuric acid or the like, and hence, is difficult to handle. In addition, the aromatic polyamide has a lyotropic liquid crystallinity, so that only a devitrified, low-strength film containing a great number of crystallites is obtained by a conventional solution-casting method.
As a process for producing a transparent, aromatic polyamide film, JP-A-62-246,719 discloses a process comprising separating as a polymer an aromatic polyamide obtained by polymerization in a solvent such as N-methyl-2-pyrrolidone or the like, subsequently dissolving the polymer in conc. sulfuric acid, moistening the resulting optically anisotropic, conc. sulfuric acid solution at a high temperature to make it optically isotropic, subjecting the same to coagulation and washing, and then heat-treating the resulting film at a temperature of 300.degree. to 500.degree. C. under tension. However, the said process has many steps and requires complicated operations. In addition, the equipment to be used must be resistant to sulfuric acid. Therefore, there is such a problem that the cost becomes very high.
The present inventors have made extensive research for solving the above problems and have consequently found that mechanical strengths such as tensile strength, tensile modulus of elasticity and the like of the aromatic polyamide film can be remarkably enhanced by allowing an isocyanate compound to act on the aromatic polyamide to complete this invention.